He literaly uses pennies (and dimes and salt water) to light up and led. While it's not really much more than a nifty home experiment for the kids, it does get me thinking there might be a practical use.

While modern day high output LED's are very bright for the amount of electricty they use. My child uses a LED flashlight in her bed at night and I can get a week out of two AA batteries running it for 10 hours a night. The issue that I dont know if they have resolved yet is the color spectirum output of the LEDs. As far as using the LED's to acutally light the tanks during the day. Unlike the non planted tanks or non reef SW tanks a reef tank or planted tank are much more sensitive to the color spectirum in relation to the output of the lights. You can have a very bright light but if its not outputing the right specitrum the lights will be useless.

I was just thinking along the lines of night lights myself. I doubt LED's could put out enough useful energy to actually grow plants. Just thinking that instead of plugging them in, some lemon juice in a cup or something similar might be made to look cool.